There are lots of good, practical reasons why you might want to know how long you're going to live.

Sure, it would be nice to know when you need to get your emotional life in order.  But the mysterious future also figures into important financial decisions that older adults have to make, often when they still feel pretty young. How on earth can you know at 60 how much money you'll need in old age if you don't know whether you'll live 15 more years or 40?

Which form of Medicare should you choose? When should you retire? When should you downsize? Your lifespan can also be important for medical decisions.  Is it time to stop screening for a particular cancer?  Would surgery do more harm than good?

Christine Arenson is a geriatrician at Thomas Jefferson University Hospital.
Courtesy of Jefferson Health
Christine Arenson is a geriatrician at Thomas Jefferson University Hospital.

Doctors are loath to make predictions, partly because they're not very good at it and partly because they know too well that life is unpredictable.  Everyone knows a health nut who died young of cancer.  We've all heard the stories of centenarians who credit their long lives to whiskey or cigarettes.

Christine Arenson, a geriatrician at Thomas Jefferson University Hospital, especially cautions against assuming that you won't live long because your parents died relatively young, or because you have health issues. She says the biggest problem she sees in how Americans think about retirement is "we aren't preparing to live long enough."

The Social Security Administration says that, if you're 65 now, you have, on average, 19 more years of life if you're a man and 21 if you're a woman.  The agency's calculator is a crude instrument based purely on averages.  You can find multiple online calculators, some of which are tied to financial products, that purport to fine-tune an individual's prediction based on such risk factors as overall health and functioning, weight, education, and family history. One even includes the age of menopause because women who enter menopause late tend to live longer.

In general, you can guess you'll live at least a little longer than average if you've embraced a healthy lifestyle and avoided chronic health problems, and less if you haven't. People with more money and more education tend to live longer, likely the result of better health choices and better access to medical care. If you're in your early 60s and you're sedentary, overweight, or   sleep-deprived and you smoke, "you'll be lucky if you make it to 75 or so …. and, if you do, you're going to be doing it with a lot of chronic conditions," said Lisa Walke, chair of geriatric medicine at Penn Medicine.

You may not be asking the right question

How long your parents lived is less important than you'd guess, experts said, unless your family members tend to be exceptionally long-lived.  For both men and women, the mother's longevity is a better predictor of their own than a father's, said Jay Olshansky, a biodemographer at the University of Illinois at Chicago and a board member of the American Federation for Aging Research (AFAR).

Steven Austad, AFAR's scientific director and a biologist at the University of Alabama at Birmingham, said that only about 25 percent of a person's lifespan is inherited. That means that the remaining 75 percent is largely up to you.

Even if your family is riddled with early deaths, it may not be because of genetic flaws.  "It may just be that everyone has bad habits," said Robb McIlvried, a Geisinger Health geriatrician.

Plus, treatments for cancer, heart disease, diabetes, and many other chronic conditions are improved over what they were in previous generations, so things that killed your grandparents or parents might not kill you.

Estimates of how much a particular bad or good habit can affect your lifespan differ.  As Austad pointed out, this is an area that's difficult to study well.  Unlike lab mice, humans don't stick to particular diets well and they may lie about how much they exercise or smoke. That makes it hard to know how substantial a role a particular behavior plays.

Lyle Ungar, a computer and information science professor at the University of Pennsylvania who helped develop a lifespan calculator meant to help people choose healthier behaviors, said quitting smoking could add about five years to a life, as could starting to work out regularly instead of having a sedentary lifestyle.  Having good relationships instead of being socially isolated can also add years.

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Just asking how long you'll live is probably the wrong question.

"The main reason to take care of yourself is not so you can live longer," Austad said. "It's so you can be healthy longer."

It’s never too late to change

The good news is that, even if you've made a lot of health mistakes, you can still bend the curve late in life.

"It's never too late to develop a healthy lifestyle," said Eric Widera, a geriatrician at the University of California, San Francisco, who helped develop ePrognosis, a calculator for assessing risk of death.  

Changing your lifestyle now can lengthen your life, Arenson said, but its more dramatic effect will likely be on the number of healthy years.

Widera said the average woman experiences 4.5 years of disability at the end of life compared with 2.9 years for men.  Healthy lifestyles can reduce that period of disability.

If you're prioritizing, you'll likely get the biggest benefit from quitting smoking.  After that, Walke said "without a doubt, it's going to be exercise. I know no one wants to hear that."

Exercise — she recommends 30 minutes five times a week — can help with mood, sleep, and blood pressure, she said.  People of any age can build muscle with weight training, and that can reduce the risk of falls.

Walke's patient  Mary Ann Lancaster Tyler is taking her doctor's advice to heart.  Tyler is 81 and has no chronic health conditions.  She climbs the stairs in her Northwest Philadelphia home several times a day and purposely chooses distant parking places so she'll have to walk farther to stores.  She's a healthy eater and reads a lot. A lively woman who is active in her community, she's planning to live to 100 and hopes her husband, Robert, 83, will, too.

Still, Walke suggested some changes in Tyler's lifestyle if she wants to reach her goal.  Tyler is planning to return to line dancing and to start swimming, so she'll get more exercise.  She's also planning to start playing pinochle again to keep her brain even more active.  "I feel like I'm spiritually grounded, so the final, absolute decision is not mine," she said.

Lisa Walke speaks with patient Mary Ann Lancaster Tyler outside of the Penn Ralston Center.
HEATHER KHALIFA / Staff Photographer
Lisa Walke speaks with patient Mary Ann Lancaster Tyler outside of the Penn Ralston Center.

Austad and Olshansky said that there are some medicines on the horizon that might lengthen life but that they're not yet proven to work.  At least in animals, a very low-calorie diet adds to lifespan, but Austad is not willing to try it.  As a college wrestler, he had experience with cutting weight, and it's not for him.  "All I did was think about food all the time," he said.  Plus, it makes heart numbers look great, but he has concerns about what it does to bone health.

Olshansky says that if you're already exercising, eating right, and not smoking, you're probably doing about as much as you can.  Eventually, age will catch up with all of us.  "If something goes wrong," he said, "it's not your fault. …

"It's easy to shorten your life. Lengthening it, that's another story."

I’m healthy.  What kind of Medicare should I choose?

Many are faced with what may be an irrevocable decision as soon as they sign up for Medicare.

Will they pick regular Medicare and buy a Medigap policy to cover copays and deductibles, or go with Medicare Advantage?  Once you choose Advantage, there may be no going back if you have — or develop  — pre-existing conditions.

Medicare Advantage typically has lower monthly costs and may cover more than Medigap plans, but copays can really add up  —  the current max is $6,700 a year —  and medical networks are often small.  If you're really sick and want highly specialized care from a specific doctor or health system, you may not be able to get it.

How should perceptions of your current and future health figure into the decision?

James  Long, a health insurance agent at Young's Insurance Services Inc. in Eagleville, likens the Medicare decision to the one you have to make about when to start Social Security payments. "There is an unpredictability, but you play the odds," he said.

If you're in good health and have reason to think you'll live a long life, he thinks, Medicare Advantage is the way to go.  Monthly costs are lower and healthy people likely won't have huge costs for co-pays.  Yes, you'll lose money in the last four or five years of life, but you'll likely save overall, he said.

Tricia Neuman, senior vice president and director of the Kaiser Family Foundation's program on Medicare policy, said it's a complicated decision.  "It's a hard call," she said.  "Nobody can really predict the future."

Some people with lower incomes may believe that they can't afford the higher premiums for Medigap insurance. Choosing Medicare Advantage doesn't seem to be that big of a deal when they're new to Medicare and healthy, said Neuman, who doesn't make specific recommendations.  When they're older, she said, their income is likely to be smaller in real terms and out-of-pocket spending will rise.  They may wish for the predictability of a monthly Medigap premium.

Even healthy people, she said, should consider worst-case scenarios.  "Anything can happen to someone at a moment's notice," she said.

Her eyes were opened when a friend on Medicare went to an emergency room after an accident while mountain biking.  He recovered from the accident, but doctors discovered a tumor at the back of his neck.

The surgeon available to him through his Medicare Advantage HMO network didn't have much experience with removing this type of tumor.  Her friend wanted a more experienced surgeon who was not in the network. He thought he could switch to regular Medicare during open enrollment but discovered that none of the Medigap plans would accept him; the tumor was a pre-existing condition.  Her friend found another Medicare Advantage plan that included the preferred surgeon and had the procedure.  After that, he switched out of Medicare Advantage.

Regardless of your current health or predicted lifespan, that's a cautionary tale if you're the kind of person who's likely to want the doctor or hospital with the best reputation in town when faced with a serious illness.

Join Inquirer journalists and celebrity keynote speaker Genie Francis for a day of education, inspiration and resources for the 55+ community on Saturday, Oct. 13, 11 a.m., Downtown Philadelphia Marriott. Register: www.philly.com/55thrive